1. Field of the Invention
The present invention relates generally to memory devices and, more particularly, to a non-volatile chalcogenide memory device.
2. Description of Related Art
Integrated circuits, typically in the form of microprocessors, microcontrollers, or other logic circuits, are used to control the functions of many modern electronic devices. For example, integrated circuits are used to control the functions of computers, telephones, and many other consumer electronics. It is generally necessary for the integrated circuits to retrieve (read) and store (write) data as they perform their functions. The data may be in the form of instructions for the integrated circuits (e.g., a program), data necessary for the execution of a program, or data generated during the execution of the program. It is preferable to store the data in memory devices which are easily accessible by the integrated circuits.
Many different types of memory devices are known for the storage of data. In selecting a memory device, the particular requirements for the data with which the memory device will be used are important. For example, several parameters such as the quantity of data, the required access time and the required storage time can play an influential role in memory device selection.
Phase change memory is a type of memory that retains stored information after power has been removed from the memory device. This type of memory is called non-volatile memory. Phase change memory uses electrically writable and erasable phase change materials that can be electrically switched between an amorphous and a crystalline state or between different resistive states. Phase change memories have been fabricated using various materials such as chalcogenide. Chalcogenide can be made from a mixture of elements including, tellurium (Te), selenium (Se), antimony (Sb), nickel (Ni) and germanium (Ge).
Chalcogenide memory can be switched between several electrical states of varying resistivity in nanosecond time periods with the input of picojoules of energy. Chalcogenide is non-volatile and maintains the integrity of the information stored by the memory cell without the need for periodic refreshing by an electrical signal. Chalcogenide memory also retains the information in the memory cell when power is removed from the device. Chalcogenide memory can be directly overwritten without first having to erase the contents of the memory cell. Another advantage of chalcogenide memory is that it can be switched to more than two different states. Depending upon the amount of electrical energy that is passed through the memory device, the memory device can be changed from an amorphous state to a crystalline state or to varying states in between. Chalcogenide material changes conductivity or resistance based upon its crystal state. For example, in the amorphous state, chalcogenide shows a lower electrical conductivity than it does in its crystalline state.
Chalcogenide memories are switched between states by passing an appropriate current through an active region of chalcogenide between electrodes or contacts. The amount of power required to switch a memory cell is dependent upon the size of the active region. A small active region requires less power than a large active region. The size of the active region can be controlled by the size of the electrical contact or electrode.
Methods of fabricating and designs for chalcogenide memories are disclosed in U.S. Pat. No. 6,031,287 to Harshfield; U.S. Pat. No. 6,111,264 to Wolstenholme et al; and U.S. Pat. No. 6,114,713 to Zahorik, all of which are hereby incorporated by reference in their entireties. As set fourth in the prior art, a typical chalcogenide memory cell relies upon fabrication techniques that endeavor to limit the minimum dimensions of the active region. The minimum size of the active region is typically controlled by the limits of photolithography and conventional semiconductor processing technology. An important factor in reducing the required current is the size of the active region. Prior attempts at making smaller device features have attempted to deposit chalcogenide materials into ultra-small holes. Unfortunately, the small holes can be difficult to fabricate and fill, potentially resulting in poor yields during manufacturing.
Chalcogenide memories are not readily driven by CMOS circuitry due to the relatively high current requirements needed to change phases. Such prior art designs can consume an excessive amount of power in order to switch states, with the active regions of the prior art memory cells having been relatively large and difficult to fabricate.
A need exists in the prior art for a chalcogenide memory with reduced power consumption, higher density and smaller device feature sizes.